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On OS X, CGL is the foundation layer of windowing system interfaces to OpenGL. Both [[Apple Graphics Library|AGL]] (Apple Graphics Library) and the [[Cocoa (API)]] (or AppKit) have interfaces to OpenGL and are logical software layers and depend on CGL for their behavior. CGL and AGL interoperate freely. CGL and Cocoa may be used together, however Cocoa classes may implicitly make changes to CGL state. Function calls from AGL and Cocoa should not be mixed.
Configuration of these surfaces is done through a pixel format selection process where different compatible layers of rendering information are combined to form a [[framebuffer]]. Examples of such layers are color buffers, [[Alpha compositing|transparency buffers]] (alpha), [[stencil buffer]]s, and [[Z-buffering|depth buffers]]. The CGL function [httphttps://developer.apple.com/documentation/GraphicsImaging/Reference/CGL_OpenGL/Reference/reference.html#//apple_ref/c/func/CGLChoosePixelFormat CGLChoosePixelFormat] is used to perform this buffer compatibility check. CGLChoosePixelFormat will, based on input parameters and their scoring policy, choose a pixel format that represents a compatible buffer configuration that is supported by the underlying renderer that will be used to process graphics commands. [[Rendering (computer graphics)|Renderers]] may be either hardware based, such that they correspond to graphics cards installed in the system or they may be software based, where the main [[Central processing unit|CPU]] of the system handles all of the graphics command processing and final [[Rasterisation|rasterization]] work.
===Handling Mac OS X heterogeneity===
When users configure their Macintosh to use a virtualized desktop, and they drag windows from one display to another, CGL handles the management of OpenGL graphics state that must be shadowed between devices to provide command processing consistency between them. Dragging a window across a Macintosh desktop between two different displays that are supported by two different renderers is known as a "Virtual Screen Change".
CGL also provides a mechanism to obtain information about the renderer that is currently in use. The primary data structure that maintains OpenGL state on Mac OS X is a [httphttps://developer.apple.com/documentation/GraphicsImaging/Reference/CGL_OpenGL/Reference/reference.html#//apple_ref/doc/c_ref/CGLContextObj CGLContextObj]. These CGL contexts can be retrieved at any time using a call to [httphttps://developer.apple.com/documentation/GraphicsImaging/Reference/CGL_OpenGL/Reference/reference.html#//apple_ref/c/func/CGLGetCurrentContext CGLGetCurrentContext]. The ''CGLContextObj'' may then be queried for specifics about the renderer that is associated with it.
===Software renderer===
Also included is Apple's in-house OpenGL [[software renderer]]. Originally, this was a simple integer package,. inIn Mac OS X 10.3, a new floating point one was introduced [https://developer.apple.com/library/mac/documentation/GraphicsImaging/Reference/AGL_OpenGL/Reference/reference.html#//apple_ref/doc/uid/TP30001068-CH2g-AGLRenderingContextIDs which ultimately replaced it]. The software renderer, though slow, is fast enough for basic applications and [https://developer.apple.com/graphicsimaging/opengl/capabilities/index.html kept feature-complete] {{Webarchive|url=https://web.archive.org/web/20140108115648/https://developer.apple.com/graphicsimaging/opengl/capabilities/index.html |date=January 8, 2014 }} with OS X's OpenGL implementation for development purposes.
==See also==
==External links==
* [httphttps://developer.apple.com/documentation/GraphicsImaging/Reference/CGL_OpenGL/Reference/reference.html CGL reference guide on Apple website (html).] - {{webarchive |url=https://web.archive.org/web/20140812200349/https://developer.apple.com/library/mac/documentation/GraphicsImaging/Reference/CGL_OpenGL/Reference/reference.html |date=12 August 2014}}
* [httphttps://developer.apple.com/documentation/GraphicsImaging/Reference/CGL_OpenGL/CGL_OpenGL.pdf CGL reference guide on Apple website (PDF).] - {{webarchive |url=https://web.archive.org/web/20140824204714/https://developer.apple.com/library/mac/documentation/GraphicsImaging/Reference/CGL_OpenGL/CGL_OpenGL.pdf |date=24 August 2014}}
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